In navigating according to signals received from GPS satellites by a GPS receiver, the signal broadcast by each satellite must first be acquired, i.e. the GPS receiver must match the phase of a replica of the code used by the satellite in order to de-spread the signal, and to do so, must also determine the carrier frequency of the signal, a frequency that is known only nominally because of shifting in the frequency due to relative motion of the satellite and the GPS receiver (the so-called Doppler shift), or because of other factors including for example a difference in the clock (essentially an oscillator) rate of the satellites compared to that of the GPS receiver.
The Doppler shift cannot be determined by a GPS receiver unless the exact time, reference location and ephemeris or almanac are known. The difference in clock rates causes what is here called a frequency error, i.e. the difference between the locally generated (by an oscillator in the GPS receiver) carrier frequency and the nominal carrier frequency. The frequency error is one part of the overall frequency shift, and must be determined to determine the Doppler shift, the other part of the overall frequency shift. The oscillator of the GPS receiver is essentially a frequency reference, and such frequency references (oscillators) are often rather low-cost oscillators, which tend to drift due to ambient temperature changes and due to aging. In practice, it is impossible to predict how much such an oscillator has drifted since last being used. Therefore, there is always an unknown frequency error when a GPS receiver is powered on.
U.S. Pat. No. 4,445,118 teaches sending a stable frequency reference to a GPS receiver to eliminate frequency errors. U.S. Pat. No. 5,841,396 teaches using a cellular carrier frequency signal (which is not stable) as such a frequency reference. U.S. Pat. No. 6,122,506 teaches using a dedicated frequency broadcast signal, and specifically the GSM Frequency Control Channel (FCCH), to eliminate frequency errors in GPS receiver. All of these ways of eliminating frequency error are based on using a carrier frequency or a corrected version of a carrier frequency, in some cases a carrier frequency derived from a cellular communication signal, and in some cases a carrier frequency provided by a special, ground-based GPS carrier reference broadcast facility.
What is therefore needed is a way of eliminating frequency error in a GPS receiver that is based on other than a carrier frequency, so as to provide a frequency reference indirectly derived from an altogether different source than in the prior art.